CBSE Class 11 Chemistry Chapter 3 Notes : Classification of Elements and Periodicity in Properties
In this chapter, we will discuss the development of the periodic table and periodic law. The arrangement of elements in the modern periodic table is in eight vertical columns (groups)and seven horizontal rows (periods) based on their atomic number. On the basis of electronic configuration, elements can be classified into four types in the periodic table viz s-block, p-block, f-block, d-block elements. More than 70-80% of the known elements are Metals. Elements located at the top in the periodic table are nonmetals and are less than 20. In a group, as the atomic number increases, the Metallic character increases. In a period, there is a decrease in the Metallic character from left to right. Also, the chemical, as well as the physical properties, vary with their atomic number. Elements of the same group exhibit similar chemical properties because of the similarvalence shell electronic configuration.
Periodic Trends In Properties Of Elements
- Atomic sizes – With the increase in atomic number the atomic radii in the group will increase. On the other hand, the atomic radii decrease from left to right in a period.
- Ionization enthalpies – Decreases down a group but increases across a period.
- Electron gain enthalpies – Tends to become less negative down a group and more negative across a period.
- Electronegativity – Electronegativity decreases down a group but increases across a period.
- Chemical reactivity – Chemical reactivity is lowest in the centerAnd is highest at the two extremes of a period.
Few Important Questions
- What is the difference between Modern Periodic Law and Mendeleev’s Periodic Law?
- What is ionic radius? What is the atomic radius?
- What is isoelectronic species? Give an example.
- Elements in the same group have similar chemical and physical properties. Why?
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